1. Field of the Invention
The present invention is related to a screw compressor capable of manually adjusting internal volume ratio and capacity (the flow rate of discharge gas) thereof and a combined screw compressor unit suitable for application in the case the suction pressure or discharge pressure varies widely when used for compressing and supplying gas for a refrigerating machine, air conditioner, gas turbine booster, natural gas pipe line, chemical process, spherical holder, etc.
2. Description of the Related Art
A screw compressor equipped with a slide valve for adjusting internal volume ratio and an unloader valve for varying the volume of gas to be sucked, is widely used as a low-noise, low-vibration, relatively low-priced compressor which can be adjusted of the compression ratio and flow rate thereof in accordance with use and operation conditions.
FIG. 5 shows schematically an example of the construction of conventional screw compressor equipped with a slide valve for adjusting internal volume ratio and an unloader valve (a slide valve for adjusting capacity, i.e. flow rate). In this example, both the internal volume ratio adjusting slide valve and capacity adjusting slide valve are driven to slide by hydraulic pressure.
In FIG. 5, reference numeral 101 is a rotor casing, 192 is a suction side bearing housing, 103 is a discharge side bearing housing, 104 is a cylinder, 105 is a rotor drive shaft, 106 is a rotor chamber, 107 is a suction port, 108 is discharge port. A male rotor and a female rotor not shown in the drawing are accommodated in the rotor casing 101 to mesh with each other and supported by bearings in the suction side and discharge side bearing housings. A drive machine such as an electric motor or engine is connected to the rotor drive shaft 105 to rotate the male rotor. The gas to be compressed is sucked from the suction port 107 to be introduced into the enclosed space between the tooth of the male rotor and that of the female rotor, then compressed as the rotors rotate to be discharged from the discharge port 108.
Reference numeral 111 is a capacity adjusting slide valve, 112 is an internal volume ratio adjusting slide valve. Both the slide valves have faces running along the outer perimeters of the male and female rotors, these faces forming part of the wall of the rotor chamber 106. The capacity adjusting slide valve 111 is fixed to the rod 113, the slide valve 111 being provided with a cut 111a for defining a radial port. The internal volume ratio adjusting slide valve 112 is fixed to a hollow rod 114 which is received in the rod 113 for sliding. The cylinder 104 is partitioned into a cylinder chamber 121 and a cylinder chamber 120 with a partition 102a of the central part of the suction side bearing housing 102, a partition 104a in the cylinder 104, and a cover 109. The rod 113 extends penetrating the partition 104a to the cylinder chamber 120 to be connected with a piston 115 for driving the capacity adjusting slide valve 111. The hollow rod 114 extends penetrating the partition 102a to the cylinder chamber 121 to be connected with the piston 116 for driving the internal volume ratio adjusting slide valve 116. The cylinder chamber 120 is divided into two rooms of left and right, into or from each of which oil is supplied or exhausted from oil inlet or outlet ports not shown in the drawing to move the piston left or right in order to slide the slide valve 111 or 112 connected to the piston 115 or 116 by means of the rod 113 or 114. The rod 113 for sliding the capacity adjusting slide valve 111 is provided with a long center hole at the right part thereof and a rod 117 is inserted into said long center hole, the rod 117 being provided with a spiral groove 123, a pin (not shown in the drawing) protruding radially inwardly from the center hole being received in the spiral groove 123, so that the movement of the rod 113 to the left or right, namely the slide of the capacity adjusting slide valve 111 is converted into the rotation of the rod 117 to be indicated by the unloading valve indicator 118.
In FIG. 5 is shown the condition when internal volume ratio is at the maximum and capacity (flow rate) is at the maximum (full load). If the piston 115 is moved to the left in this condition, the capacity adjusting slide valve 111 is moved to the left to move away from the mating plane 119 of both slide valves and there develops a clearance between the end faces of both slide valves. Apart of the gas sucked and filled in the space between the teeth rotors leaks out, before the space is shut by the right side edge line along the outer perimeter of the teeth of the rotors to enclose the gas, through the clearance to a room 122 to be returned to the suction port 107 through a passage not shown in the drawing. Therefore, the volume of the gas enclosed in the space between teeth is reduced and the amount of discharged gas is reduced.
If both the slide valves are moved to the right by hydraulic pressure with both slide valves contacting, the cut 111a of the capacity adjusting slide valve 111 enters the rotor chamber, so the radial port is opened. The more the slide valves moved to the right, the faster the radial port opens. So the more the slide valves moved to the right, the smaller becomes internal volume ratio. When hydraulic pressure is applied on the right side of the piston 115 to move the capacity adjusting slide valve 111 to the left in the condition internal volume ratio is small, the capacity adjusting slide valve 111 is moved to the left to move away from the mating plane 119 of both slide valves and there develops a clearance between the end faces of both slide valves, and the amount of discharge gas is reduced by the same reason mentioned before. With the construction like this, capacity (flow rate) can be adjusted with arbitral inner volume ratio.
According to the construction of above example, both the internal volume ratio adjusting slide valve and the capacity adjusting slide valve are slid by hydraulic pressure, however, there is a type in which the internal volume ratio adjusting slide valve is screwed on a rod extending through the capacity adjusting slide valve and the internal volume ratio is adjusted by rotating the rod by means of a step motor to slide the internal volume ratio adjusting slide valve. There is also a type in which said rod is rotated manually when the operation of the compressor is stopped and fixed at a appropriate rotation position by a lock nut to secure the internal volume ratio adjusting slide valve in place.
With the conventional screw compressors equipped with an internal volume ratio adjusting slide valve and a capacity adjusting slide valve mentioned above, internal volume ratio and capacity can be adjusted automatically or internal volume ratio can be adjusted manually with the operation of the compressor stopped. Such a compressor can be used for a variety of uses, however, the construction is complicated, which causes increase in cost. Further, an electric power source is required to drive an oil pump or step motor for generating the hydraulic pressure to move the slide valves. Therefore, there is inconvenience that such a compressor can not be used as it is in wild land where electricity can not be available. For example, in natural gas fields, screw compressors are driven by gas engines using extracted natural gas as fuel, and it is troublesome in many cases to provide a electric power source for adjusting internal volume ratio and capacity of the screw compressors.
On the other hand, capacity controllable screw compressors have been used widely for refrigerating machines. A plurality of compressors have been combined to compress gas through a plurality of compressors, for example, two or three compressors to reduce the compression ratio per one stage for improving compression efficiency, for polytropic efficiency is low if it is intended to attain high compression ratio (ratio of discharge pressure to suction pressure) by a single compressor.
Generally, in a screw compressor, the internal volume ratio is determined in the design stage, and a compressor of proper internal volume ratio is selected among compressor specifications of low, intermediate, and high compression ratio depending on uses. The selected compressor achieves maximum polytropic efficiency under a certain operating condition, i.e. at a certain compression ratio, and polytropic efficiency decreases at compression ratios other than that. This is for the wasteful work needed to be done when the compressor is operating at the compression ratio other than the compression ratio corresponding to the internal volume ratio of the selected compressor, because a pressure difference is developed between the pressure in the discharge space and that of the gas to be discharged into said space from the compression space formed by a pair of rotors of the compressor.
There have been developed screw compressors capable of adjusting internal volume ratio and capacity, however, they are inevitably complicated in structure and high in cost as mentioned above.
When a plurality of conventional compressors with constant internal volume ratio, for example, two of such compressors are combined to attain high compression ratio, one is a lower pressure compressor and the other is a higher pressure compressor. The lower pressure compressor compresses sucked gas at the compression ratio corresponding with the design internal volume ratio determined in the design stage of the lower pressure compressor and discharges the compressed gas to the inlet side of the higher pressure compressor.
The higher pressure compressor compresses the gas discharged from the lower pressure compressor at the compression ratio corresponding with the design internal volume ratio determined in the design stage of the higher pressure compressor.
Therefore, the suction pressure of the higher pressure compressor (intermediate pressure) depends on the ratio of the volume of the enclosed space between teeth of the lower pressure compressor when discharge from the space begins to the volume of the enclosed space between teeth of the higher pressure compressor when compression begins, i.e. the volume of the maximum enclosed space between teeth of the higher pressure compressor.
To be more specific, if the volume of the enclosed space between teeth of the lower pressure compressor when discharge begins is smaller than the volume of the enclosed space between teeth of the higher pressure compressor when compression begins, the gas discharged from the lower pressure compressor is enclosed in the space between teeth which is larger than the space between teeth of the lower pressure compressor when discharge begins, so that the pressure of the gas when compression begins in the higher pressure compressor is lower than that when the gas is discharged from the lower pressure compressor. That is, the intermediate pressure (suction pressure of the higher pressure compressor) becomes lower than the discharge pressure of the lower pressure compressor. Therefore, the gas discharged from the lower pressure compressor expands in the space between the lower pressure compressor and higher pressure compressor, that means that the lower pressure compressor compresses the gas excessively high and does wasteful compression work, resulting in decreased efficiency of the lower pressure compressor.
Now if we call the ratio (the volume of the enclosed space between teeth of the lower pressure compressor when discharge begins)/(the volume of the enclosed space between teeth of the higher pressure compressor when compression begins) as displacement ratio, the smaller the displacement ratio is, the lower the intermediate pressure becomes, resulting in excessively high compression in the lower pressure compressor.
It is desirable to operate the combined compressor unit so that said displacement ratio is kept to be 1 or slightly smaller than 1 to evade large pressure drop when the discharged gas from the lower pressure compressor enters the suction port of the higher pressure compressor.
The discharge pressure of a screw compressor is (Vi)m times the suction pressure, where Vi is internal volume ratio, and m is polytropic exponent. Assuming polytropic exponent m is 1.3, when design internal volume ratio is 2.5, discharge pressure is 3.29 for suction pressure of 1.0, 4.94(=3.29×1.5) for suction pressure of 1.5, and 6.58(=3.29×2) for suction pressure of 2. If these discharge pressure of the lower pressure compressor are the suction pressure of the higher pressure compressor, and assuming polytropic exponent m is 1.3 and design internal volume ratio is 2.5 also in the higher pressure compressor, discharge pressure of the higher pressure compressor is 10.8, 16.2, and 21.6 for suction pressure of the lower pressure compressor of 1, 1.5, and 2 respectively.
As described above, when the suction pressure of the lower pressure compressor increases, the discharge pressure of the higher pressure compressor increases considerably, and there happens the case that the discharge pressure exceeds the limit pressure permissible for the higher pressure compressor, which may induce damage of the components of the higher pressure compressor.
When the displacement ratio is small, the intermediate pressure, i.e. the suction pressure of the higher pressure compressor becomes lower than the discharge pressure of the lower pressure compressor (the pressure in the enclosed space between teeth just before discharge begins), but even so, the discharge pressure of the higher pressure compressor may happen to exceed the permissible pressure when suction pressure (the suction pressure of the lower pressure compressor) is highly increased. The larger the design internal volume ratio is, the stronger this tendency is.